Why Does an Electric Tester Not Work in DC Circuits?

Electrical Technology

Why Does an Electric Tester Work Only in AC, but Not in DC Circuits?

Electric testers, often referred to as voltage testers, phase testers, mains testers or test pens, are essential tools for electrical engineers and technicians. These devices are designed to use as phase tester to detect the presence of voltage in electrical circuits, providing a quick and simple way to ensure safety and verify live wires.

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Principle of Operation of AC Phase Tester

The basic AC contact phase tester contains on a neon bulb and a high impedance resistor connected in series to the metallic body of tester. When the metallic part make a contact with phase wire, the resistor inside the tester reduces the 120V or 230V AC to the operating voltage of neon bulb which is 80V.

The circuit is completed through the tester to human body standing on earth or touching a wall (which acts as neutral) to the earth. The neutral in an AC supply is connected somewhere to the ground either at transformer or substation side. Hence, the circuit is completed, the tester detects the AC voltage (i.e. phase wire) and the neon bulb inside the phase tester glows.

Most electric testers, particularly non-contact voltage testers (NCVTs), operate on the principle of capacitive coupling. They detect the electric field generated by a live wire. They only work in AC circuit like transformers and won’t operate in the DC circuits like 12V car battery.

Why Does an AC Phase Tester Not Detect DC Supply?

While AC electric testers are highly effective in detecting AC (alternating current) voltage, they often fail to work properly with DC (direct current) voltage due to the following reasons.

Absence of Neutral

In Case of AC Circuit

In an AC circuit, when an operator touches the phase tester to the phase (live) wire, the current flows through the high-impedance resistor, neon bulb, and human body to the ground. Since the neutral is connected to the ground at some points (e.g., at the transformer where the neutral is grounded and connected to the earth), the circuit is completed, and the neon bulb inside the AC phase electric tester glows.

In Case of DC Circuit

As we know, in AC circuits, there is a phase and neutral where the potential difference at the phase wire is 120V or 230V AC, while the voltage level at the neutral side is 0V. On the other hand, in DC circuits, there is positive and negative. For example, in a 12V battery, the voltage level on the positive (+) terminal is +12V, while the voltage level at the negative (-) terminal is -12V. Since there is no real ground, the circuit won’t complete even if you touch the phase tester to the positive or negative terminal of the battery. Therefore, unlike the AC circuit, the electric tester won’t detect the DC voltage, and the neon bulb won’t glow.

Minimum Striking Voltage

In Case of AC Supply

The striking voltage for a neon bulb inside the phase tester is 80V. When the tester makes contact with a phase wire in a 120V or 230V AC circuit, the series resistor in the tester circuit drops the voltage to the desired level, which ionizes the gas chamber of the neon bulb. Consequently, the bulb glows.

In Case of DC Supply

The 12V DC is insufficient to ionize the gas inside the neon bulb glass chamber; hence, the bulb won’t glow. To illuminate the neon bulb inside the phase tester, a minimum of 80V is required, whether AC or DC.

Will the Neon Bulb Glow If the DC Voltage is Raised to 80V?

Yes, the neon bulb inside the tester will glow if the voltage level is 80V (either AC or DC), but a proper neutral connected to the ground is needed to complete the circuit. In the case of an 80V DC supply, the jumper wire between two batteries connected in a series circuit should be grounded, i.e., connected to the earth. In the case of a single battery, the negative terminal should be grounded to detect the DC voltage and illuminate the neon bulb inside the electric tester.

Which Tester Should Be Used for Detecting and Measuring the DC Supply?

There are different contact voltage meters and NCVTs (non-contact voltage testers) available in the market that work with both AC and DC circuits, such as car batteries and transformers. These testers are used to measure AC/DC quantities and parameters and detect the phase in AC or the positive terminal of a battery. Some of the following tools are used to test DC circuits:

Multimeter: For testing DC circuits, you need a multimeter. These are more sophisticated tools with different settings that allow you to measure both AC and DC voltage, current, and resistance..
Contact Voltage Meters: Unlike non-contact testers, contact voltage meters can directly measure DC voltage by making a physical connection with the circuit. These meters typically use a different detection mechanism involving resistive voltage dividers and precision amplifiers to provide accurate DC readings.
Hall Effect Sensors: Some advanced testers incorporate Hall Effect sensors, which can detect the presence of a magnetic field generated by current flow, regardless of whether it is AC or DC. These sensors are more complex and expensive but offer the advantage of detecting both types of currents.
Dedicated DC Testers: Instruments specifically designed for DC applications are available and are necessary for accurate DC voltage detection. These testers utilize different principles, such as direct contact measurement and electromagnetic detection, to ensure reliable performance in DC circuits.